In accordance with the prior art, a detector normally monitors an electrical control circuit so that if one circuit element opens (such as a pressure switch contact indicating high gas pressure) the process, machine, or whatever is controlled by the circuit, will shut-down. Often the first element that shuts the process down may not be known. For example, the gas pressure may have only been too high for a moment, quickly returning to normal, causing the gas switch to close and to operate normally. Thus, the operator may not be apprised of the problem causing shut-down. This has the disadvantages of causing unnecessary shut-downs and in causing unnecessary time and effort in locating the true cause of the shut-down.
In order to eliminate this problem early prior art fault detectors used relays to hold and lock out other fault inputs after detection of the first fault, insuring that only the first fault is displayed. More recent designs use solid state logic for this purpose, such as set-reset elements and "D" type flip-flop circuits.
It is therefore the principal object of this invention to provide an improved first fault detector and monitor which will hold and display the "first fault" to cause shut-down, until the monitor is reset.
More particular objects of the invention are to provide a method or apparatus for detecting a first fault employing a simplified circuit, and at a lower cost.
Assume that there is a series of N switches to be monitored by the "first fault" locator system of the present invention. The N switches to be monitored, may for example, be connected in series operating relation with a single source of power, or they may be energized from separate sources of power. Each monitored switch is connected in shunt across the terminals of a separate, electrically isolated input of the monitoring circuit. Thus, there are N isolated monitoring inputs to correspond to N switches being monitored.
In accordance with the present invention, a triggering optical signal means is connected across each of the isolated input circuits. Each of these triggering means is optically, but not electrically, coupled to a corresponding light responsive device for each circuit which, when optically triggered, becomes conducting at a triggering voltage V.sub.t whenever the triggering optical signal means in the respective circuit lights up indicating a detected fault in its corresponding monitored switch.
When the light responsive device becomes conducting in response to a detected fault, it energizes a display signal device for indicating the fault in the specific circuit, which may take the form of an optical signal means.
At the same time, the output of the conducting light responsive device is impressed on one of a plutality of stacked gating circuits causing an output signal to be generated from the gating circuits which then passes through an amplifying and voltage reversing circuit. The resulting negative voltage signal V.sub.n is impressed on the triggering electrode for each of the light responsive devices. This has the effect of disabling each light responsive device which is not yet conducting, thereby preventing it from being triggered to become conducting in response to an optical signal from its corresponding isolated input circuit. This occurs because the voltage across the light responsive device is less than the triggering voltage V.sub.t, which allows it to become conducting. However, the negative biasing voltage V.sub.n is ineffective to shut off the light responsive device in the "first fault" indicator, because the voltage thereacross is greater than V.sub.c, the minimum voltage which allows it to remain conducting. Hence, the optical signal device representing the first detected fault continues to stay lighted until a springbiased reset button is depressed to turn it off, cutting off the direct current operating voltage V.sub.dc to it and the remaining disabled light responsive device, permitting them to be reset.
In a specific circuit embodiment disclosed by way of illustration, the isolated input circuit connected across each of the switches to be monitored comprises a triggering light-emitting diode in parallel with rectifying means. The triggering light-emitting diode across each separate input is optically coupled (but electrically isolated) with respect to a light responsive device comprising a silicon controlled rectifier, individual to each of the separate monitoring circuit inputs. When the silicon controlled rectifier becomes conducting, it energizes a display light-emitting diode (LED). The anode of the light responsive silicon controlled rectifier in the individual input circuit is also connected to one input terminal of a series of stacked gates, the other input terminals of which are connected in similar relation to the anodes of the light responsive silicon controlled rectifier circuits in each of the remaining input monitoring circuits for each of the N switches to be monitored.
The output of the stacked gating circuits is connected through an inverting amplifier and applied to the gating electrode of the silicon controlled rectifier in each of the monitoring input circuits to impose a disabling bias thereon.
The circuit operates as follows:
Assume that one of the monitored switches opens. This puts the line voltage across the isolated input of the monitoring circuit corresponding to the open switch, causing the triggering light-emitting diode (LED) to light up, and through optical coupling, to turn on the current in the silicon controlled rectifier (SCR). This causes the display LED to light up on the indicator panel, indicating a fault, and at the same time causes a high on one leg to the input of the corresponding NAND gate, causing the output of the stacked gating circuits to go low. The output from the stacked gating circuits is imposed on the input of a reversing amplifier, which functions to impose a negative bias (e.g. -12 volts) on the gating electrode of the silicon controlled rectifiers (SCR) in each of the isolated monitoring circuits. This negative bias serves to lock out and prevent any one of the silicon controlled rectifiers, which has not been conducting, from turning on; but it does not turn off the current in the silicon controlled rectifier in the first fault responsive circuit, whereby that circuit remains operated, and continues to energize the signal display light indicating a first fault, which remains lighted on the front panel indicating in which circuit the first fault has occurred.
Once the fault has been corrected, a reset, spring-biased push button enables the positive twelve volts to be cut off, so that the silicon controlled rectifier in the `first fault` circuit ceases to conduct, turning off the signal lamp. Simultaneously, the lockout bias is removed from the silicon controlled rectifier in each of the remaining monitoring inputs, thereby setting up the circuit for reoperation.
In a modified embodiment, instead of a display signal lamp to each isolated input, the fault signals are decoded and fed into a single display device, such as a digital indicator.
It will be understood that one or many circuits may be simultaneously monitored using a method and apparatus in accordance with the present invention. Moreover, the monitoring system can be set up so that the monitored circuits may be powered from a single source, or from independent sources; and out-of-service contacts may also be included in those monitored.
Particular features of the first fault locating system of the present invention are that each of the input circuits is electrically isolated; it triggers and holds operated a display device, upon location of a "first fault"; and it locks out other inputs from subsequent fault occurrences.
Particular advantages of the present invention are its simplicity, and the lock-out feature which prevents turn on of all other light-responsive elements in the monitoring input circuits, except the "first fault" locating input, which remains conducting, and which cannot be turned off, except by actuation of the reset push button. Another advantage is that any desired number of inputs may be stacked.
These and other objects, features, and advantages will be better understood from a study of the detailed specification hereinafter with reference to the attached drawings.